As is well known, an atom of any element is made up of a nucleus, with an electron cloud surrounding the nucleus. Electrons of the electron cloud carry a net negative charge, and the nucleus carries a net positive charge. The nucleus is further made up of nucleons; i.e. protons having a positive charge and neutrons having a no charge. In the nucleus, the protons and neutrons are attracted together by the strong force, which overcomes the electromagnetic repulsion between the positively charged protons. While sufficiently strong so as to attract protons and neutrons tightly into a nucleus, the strong force is only effective over a very small distance, on the order of 1 or 2 nucleon diameters. This limits the maximum size a nucleus can attain; lead 208 is the largest known stable nucleus having 208 neutrons and protons. Atomic nuclei containing more than 208 neutrons and protons are generally unstable, and decompose by shedding neutrons, protons and “quantums” of binding energy, typically gamma photons, representative of forces that temporarily held the released protons and neutrons to the unstable nucleus.
Other ways a nucleus can become unstable is for one or more extra nucleons to be introduced into the nucleus, creating an unstable nucleus. For example, it is well known that any combination of 5 nucleons is extremely unstable, and such a nucleus will rapidly decompose into one or more stable nuclei of stable configurations by the emission of one or more nucleons and energy. Of particular interest is the reaction of two isotopes of hydrogen, deuterium and tritium. Deuterium is a hydrogen atom, which typically has a single proton as the nucleus, and to which a neutron is added. This nucleus is called a deuteron. Tritium is a hydrogen atom to which two neutrons are added (called a triton). While these two atoms naturally repel each other due to electromagnetic repulsion of the protons, when brought close enough together, as by accelerating one nucleus into the other, the strong force becomes effective to cause the two nuclei to temporarily fuse together into a compound nucleus before decomposing. The decomposition or decay reaction may be symbolized as:T+D→4He+NMeaning that the unstable nucleus formed by a deuteron and triton decomposes into a helium 4 ion and a neutron having an energy of about 14.1 MeV (mega electron volts). The binding energy of the unstable nucleus is released as a gamma ray photon. Similar reactions takes place when two deuterons are combined, this reaction isD+D→3He+N and D+D→1H+3HMeaning that a helium 3 ion and a neutron having an energy potential of about 2.5 MeV are produced, along with the corresponding gamma ray.
Conventional neutron generators of the prior art relevant to this invention may typically use a tritiated target, or in some instances a deuteriated target. Such a target may take the form of a metal hydride imbedded or containing tritium or deuterium. A small supply of deuterium gas provides a gas feed that is fed at a very low rate first through an ionizing electrical field to ionize individual atoms of deuterium (stripping off one or more electrons from the nucleus), creating deuterons that have a net positive charge. After being ionized, the positively charged deuterons may then be focused and accelerated to an energy of about 100-110 KeV using electrostatic fields into a beam of ions that is directed at the tritiated or deuterated target. 100-110 KeV is an energy level that maximizes a probability that a deuteron will fuse with a tritium nucleus. When deuterium is accelerated into deuterium, a somewhat higher accelerating voltage (110-150 KeV) is required to maximize the neutron output. In the target, the high energy deuterons undergo collisions with the target deuterium or tritium atoms and fuse therewith to temporarily create an unstable compound nucleus that immediately decays as described above. Neutrons that are produced by DT or DD collisions are emitted isotropically, that is, the neutrons are emitted equally in all directions, with no preference to the direction of emission. As neutrons have no charge, they cannot be controlled in the same manner as electrons and other charged particles. Neutrons produced in such a manner may then be used to irradiate elements of the subject under scrutiny and cause radioactive activation of these elements. For purposes where deep penetration by the neutrons is desirable, neutron generators using DT reactions producing relatively high energy neutrons is preferential, while in applications such as materials or nondestructive analysis, neutron generators using DD reactions that produce lower energy neutrons may be used.
In the neutron activation analysis technique currently in use, and as noted, an isotropic neutron source is brought within close proximity to a subject or sample to be analyzed to determine its elemental composition. Such proximity typically is on the order of a few inches to at most, a few feet. The relatively small number of neutrons that happen to irradiate target atomic nuclei cause emission of a unique spectrum, or signature, of gamma rays for each element. In this method, measurements are made of gamma rays that are either emitted almost instantaneously (prompt gamma-rays), or gamma rays that are delayed. Prompt gamma-rays are emitted essentially instantaneously from inelastic scattering, and are emitted from a compound nucleus formed when a neutron is captured by a target nucleus in the sample. Delayed gamma rays, on the other hand, are emitted by radioactive decay of one or more unstable intermediate nuclear states formed when an elemental atomic nucleus captures one or more incident neutrons. Analysis of the composite emitted gamma ray spectrum from these events allows a precise determination of the elemental content of the sample.
Where interest lies in detecting explosives, the presence of explosive compounds may be reliably detected utilizing the technique of irradiating the explosive with neutrons and observing the gamma rays produced by inelastic scattering, thermal neutron capture, and neutron activation. As the vast majority of explosives contain high concentrations of carbon, nitrogen and oxygen, strong gamma ray signatures of these elements together due to irradiation by neutrons may be taken as an indication of the presence of explosives. This technique of identifying elements by their gamma ray signature has been researched and well-developed for more than ten years (Ref. 1, 2). However, this technique has a serious drawback that limits the effective range at which the explosives can be detected (Ref. 3).
Neutron-based explosive detection systems of the prior art have used accelerator-based neutron sources, radioisotopes, or nuclear reactors (Ref. 4). These systems all suffer from the same problem in that they generate their neutrons isotropically, that is, there is no preferred direction in which the neutrons are generated. The neutron flux is equal in all directions. Thus, the vast majority of neutrons travel in directions other than toward the target and strike, among other elements, carbon, oxygen, nitrogen, and hydrogen atoms in the surrounding environment, creating large amounts of background noise. This noise limits the detection range for currently developed systems to between a few inches and a few feet, depending on the quantity of explosive being observed. As should be apparent, the need for locating or orienting the neutron source so close to the explosives is to put a sufficient number of neutrons into the explosives to cause a gamma ray signature of the explosive to stand out from the background noise.
Current accelerator-based neutron generators produce their neutrons isotropically because at the moment of fusion of the deuterium and tritium nuclei, the spins of the nuclei are randomly oriented. Research performed in the early 1960's demonstrated that the angular distribution of fission fragments emitted by neutron induced nuclear fission is not a random isotropic distribution, but rather is completely determined by the initial conditions of neutron and nuclei spins coupled with the total angular momentum.
The same principles of conservation of spin, angular, and linear momentum may be applied to the fusion of deuterium and tritium nuclei and the corresponding angular distribution of the neutrons and alpha particles resulting from the fusion reaction. A paper (Ref 5) entitled “SPIN-POLARIZED COLLISION OF DEUTERIUM AND TRITIUM: RELATIVISTIC KINEMATICS”, by Thomas B. Bahder and William C. McCorkle., crediting William V. Dent, Jr. (Applicant) and dated Apr. 17, 2008, published by the Charles M. Bowden Research Facility, Weapons Sciences Directorate, Army Aviation and Missile Research, Development and Engineering Center at Redstone Arsenal in Huntsville, Ala., this paper being incorporated in its entirety by reference herein, examines the conservation of four momentum and conservation of intrinsic spin were considered in the context of special relatively. The deuterium nucleus, with a spin magnitude of 1, is oriented in an up direction, while the tritium nucleus, with a spin magnitude of ½, is oriented in a down direction at the moment of fusion. For a deuterium nucleus of energy 107 keV, the energy for maximum cross section for fusion and striking a stationary tritium nucleus, two solutions arise with the resulting emission of neutrons at plus and minus 82.85 degrees from the incident beam axis. In other words, if the nuclear spins of both the deuterium and tritium nuclei are aligned at the moment of fusion, the coupling of the spin, angular, and linear momentums should cause neutrons to be emitted in a pair of relatively tight beams, one beam being +82.85 degrees with respect to the deuterium ion beam, and the other beam being −82.85 degrees with respect to the deuterium ion beam. A pair of corresponding alpha particle beams are emitted in an opposite direction with respect to the neutron beams. While the incorporated paper ends with a conclusion that non-zero impact parameters will lead to orbital angular momentum in the final state of the deuterium and tritium nuclei, Applicant believes this distribution of velocities will be insufficient to diverge the neutron beams to an unusable extent as compared to currently available isotropic neutron sources.
By way of example, a neutron beam generator of the instant invention may be mounted on a vehicle, and the neutron beam scanned back and forth so as to scan the ground in front of the vehicle in order to detect buried explosives while the vehicle is some distance away from the explosives. Here, a neutron generator of the instant invention may be mounted in scanning gimbals in order to scan and point the entire neuron generator, and thus the neutron beam, in desired directions. In this type application, the lack of background noise that otherwise would be produced by isotropic neutron emission would greatly increase detectability of gamma ray signatures indicative of explosives.
In addition to conventional explosives, nuclear materials may also be detected. For example, uranium 235, 238, plutonium and other radioactive materials exhibit strong gamma ray signatures when struck by neutrons.
Other applications include equipment for rapidly scanning containers as they are loaded onto or offloaded from ships or truck carriages, airport and border crossing security systems, or possibly airborne scanning and/or pointing systems for remotely detecting materials in or on the ground. As should be apparent to those skilled in the art, upon development of apparatus that generates at least one relatively tight neutron beam, many other applications will result.
The key technical issue for this invention is the production of neutron beams produced and emitted directly from a target. Directionality of the neutron beams is determined by direction of nuclear spin orientation of deuterium ions in the beam and spin orientation of deuterium and tritium nuclei in the target at the moment of fusion. For instance, deuterons in an ion beam directed to a deuterium or tritium target may be oriented with their spin alignments pointing up, while deuterium or tritium nuclei of the target may be oriented with their spin alignments pointing down (anti-aligned). In this instance, the Bahder et. al. paper incorporated herein by reference predicts generation of two neutron beams, one at +82.85 degrees and the other at −82.85 degrees, each with respect to an axis of the deuteron beam. Thus, it should be possible to directly steer the neutron beams through an arc by synchronously varying direction of spin orientation of both the deuteron beam and target nuclei, keeping the spin axis of both the deuterons and target nuclei parallel. In practice, any sweep angle should be possible by synchronously varying spin angles of the deuterons and target nuclei by a selected amount. It may also be possible to vary direction of spin alignment of one of the deuteron beam and target nuclei in order to sweep the neutron beams in a selected arc.
The physics of nuclear magnetic spin alignment is very well known and practiced every day by the nuclear magnetic resonance imaging (MRI) industry. However, magnetic fields of MRI machines spin align only a very small fraction of hydrogen nuclei in a patient undergoing observation. Also, MRI machines observe spin of normal hydrogen, which has a spin value of ½. Tritium also has a spin of ½, which splits into two magnetic sublevels: mI=+½ and −½. Deuterium, on the other hand, has a spin of 1, with magnetic sublevels: mI=+1, 0, and −1. As noted, to generate a beam of neutrons, deuterons of an ion beam and deuterium or tritium nuclei of the target must each have their spins fixed at a selected orientation at the moment of fusion.
Production of a highly spin polarized beam of atomic deuterium (Ref. 6) has been performed at a number of nuclear physics facilities for more than 10 years for experimental purposes. For instance, a paper entitled SPIN-EXCHANGE EFFECTS ON TENSOR POLARIZATION OF DEUTERIUM ATOMS (Ref. 7), by H. J. Bulten, Z. L. Zhou, J. F. J. van den Brand, M. Ferro-Luzzi and J. Lang, published in THE AMERICAN PHYSICAL REVIEW, vol. 58, no. 2, pgs. 1146-1151, (August 1998) describes an ion polarimeter diagnostic instrument to measure the tensor polarization of polarized deuterium. In this case, a small amount of polarized deuterium gas was extracted from a polarization cell. The gas was ionized by an electron beam and accelerated to 60 keV and fired into an unpolarized tritium target. An expression for the angle-dependent neutron emission rate is given in Ref. 7 for the case of fusing polarized deuterium with unpolarized tritium absorbed into a titanium disk. However, this paper does not show the case of polarized deuterium being accelerated into a target containing polarized tritium or deuterium nuclei. While this paper does show a slight anisotropy of neutron production, it does not show a strong anisotropy of neutron production due to tritium in the target being unpolarized.
Nuclear spin polarized targets are known (Ref. 8-10). For instance, another paper (Ref. 11) entitled LASER-DRIVEN NUCLEAR POLARIZED HYDROGEN INTERNAL GAS TARGET, by J. Seely et al, published in THE AMERICAN PHYSICAL SOCIETY, A 73, 062714 Pgs1-14, (2006), and which is incorporated herein by reference, describes a polarized hydrogen gas target which is used in scattering experiments. Here, apparatus is disclosed wherein deuterium ions are passed through a rubidium or potassium vapor cell. The electrons associated with the rubidium or potassium vapor are spin polarized by optical pumping with a circularly polarized laser tuned to the n=3 to n=2 transition in the alkali vapor. Potassium or rubidium is chosen because of the relatively high charge exchange cross section with fast deuterons, and the readily available tunable Ti-sapphire lasers or diode lasers with high power at the required wavelength. In this vapor cell, the deuterium ions pick up a spin polarized electron from the rubidium or potassium atoms, and while becoming neutralized, also become spin polarized.
The deuterium ions pick up a spin polarized electron primarily into the n=2 excited state To preserve the polarization state after neutralization, the alkali vapor cell is contained in a magnetic field. This magnetic field preserves the spin polarization state as the deuterium atom decays to the ground state after the charge exchange has occurred. As the spin polarized deuterium atoms emerge from the vapor cell, the atoms enter a second ionizer to allow acceleration and current measurement. The nuclei first pass through a pair of sextupole magnets to separate the spin states according to the Stern-Gerlach principle, passing a single spin state, such as +1. The ions then pass through a sextupole magnet, and their polarization measured.
Applicant proposes that when deuterons are fused with tritium or deuterium nuclei, if the nuclear spins of both the deuterons and target nuclei are fixed in selected spatial orientation just prior to the moment of fusing, then the resulting production of neutrons and alpha particles (for the case of deuterium and tritium fusing) or the resulting protons and tritium nuclei or neutrons and helium 3 (for the case of deuterium fusing with other deuterium nuclei) that these resulting particles will be emitted in a distribution directly from the target with a high degree of anisotropy, which should be on the order of 3:1 or better. It is also believed an anisotropy of at least 10:1 or better is achievable. As noted, it may be possible to adjust directionality of the neutron beam by adjusting spin alignment orientation of either deuterons of the beam, adjusting spin alignment orientation of deuterium or tritium atoms of the target, or perhaps by adjusting both. In other words, a neutron beam produced by the instant invention may be steered by controllably adjusting or varying spin alignment of deuterons of the ion beam or by controllably adjusting or varying spin alignment of deuterium or tritium atoms of the target, or perhaps both.